The present invention has particular application to spark ignition internal combustion engines, and shall be described in relation to gasoline-fuelled internal combustion engines, but may be applied to similar engines such as those fuelled by alcohols, alcohol-gasoline blends and liquefied petroleum gas (LPG). The currently regulated emissions for gasoline engines relate to unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). Whilst aftertreatment of the exhaust gases by the traditional three-way catalyst (TWC), combined with engine management of air-fuel ratios, achieves useful reductions in all of these pollutants, the progressive tightening of regulations throughout the world has posed some problems. For example, on start-up and before the TWC “light-off” (i.e., before the TWC reaches its operating temperature), HC emissions are effectively untreated. “Light-off” may be described as the temperature at which a catalyst catalyses a reaction at a desired conversion activity. For example “CO T50” is a temperature for a given catalyst at which the catalyst catalyses the conversion of carbon monoxide in a feed gas, e.g., in an exhaust gas, to carbon dioxide with at least 50% efficiency. Similarly, “HC T80” is the temperature at which hydrocarbon, perhaps a particular hydrocarbon such as octane or propene, is converted to water vapour and carbon dioxide at 80% efficiency or greater.
HC emissions from the tailpipe in the pre-light-off period are preferably prevented from leaving the exhaust system. One proposal for achieving this objective is to absorb HC emissions in an absorbent material or “HC trap” during this period. For example, JP 8-10566 discloses a catalyst-adsorbent in which a catalyst material effective for decreasing CO, HC, and NO, in internal combustion engine exhaust is combined with an adsorbent material that traps hydrocarbon during cold discharge start-ups. The trapped HC is subsequently released from the trap material for treatment (i.e., conversion into CO2 and water) by the TWC after the TWC has reached its HC light-off temperature. Also, in JP 62-5820, an absorbent is used in combination with a catalyst so that hydrocarbons exhausted with the exhaust gas at low temperature are absorbed by an absorbent, while at high exhaust gas temperature the hydrocarbons exhausted from the engine are purified, together with the hydrocarbons released from the absorbent, by the catalyst.
Typical temperatures of gasoline engine exhaust gas upstream of catalytic converters are generally over 800° C., and may be appreciably higher. Moreover, the exhaust gas temperatures may also be raised by passage through catalysts such as a small start-up catalyst, close-coupled to the exhaust manifold. Thus, the materials proposed for use as HC absorbents in this type of arrangement require high temperature stability, examples of which include gamma alumina, porous glass, activated charcoal or the like.
However, these materials are not sufficiently absorptive of HC, and lose much of the absorptivity at high temperature. When the exhaust gas temperature is in a range between the temperature at which the absorptivity starts to decrease and the temperature at which the purification by the catalyst starts to be available, hydrocarbons are exhausted with neither absorption by the absorbent nor purification by the catalyst. Thus, a conventional HC trap provided on the upstream side of a catalytic converter is not very effective, in that the hydrocarbons absorbed by the absorbent are again released before the catalytic converter provided on the downstream side thereof becomes active, thereby allowing such hydrocarbons to be emitted to the atmosphere without being purified.
In contrast to the abovementioned high-temperature materials, zeolites are known to have excellent HC absorption properties. In addition, various methods are known to improve the performance of HC adsorption and zeolite stability. For example, JP 08-099033 discloses a HC trap comprising silver, a group IIIB element e.g. cerium, lanthanum, neodymium or yttrium, and a zeolite. The silver improves HC adsorption, particularly relatively high temperature HC adsorption, and the group IIIB element improves the hydrothermal stability of the zeolite.
Nevertheless, most useful zeolites are not stable at typical exhaust gas temperatures for spark-ignition engines. To compensate for the decreased temperature stability, HC traps comprising zeolite materials are conventionally positioned downstream of a TWC so that the exhaust gas temperature cools before contacting the HC trap. However, such arrangement requires additional exhaust system components, such as an oxidation catalyst placed further downstream of the HC trap to convert the released hydrocarbons. (See, e.g., U.S. Pat. No. 6,074,973, disclosing a catalyzed HC trap comprising silver dispersed on zeolites, such as ZSM-5, wherein the HC trap is disposed downstream of a TWC.)
It also has been proposed in SAE paper 2007-01-0929 “HC Adsorber System for SULEVs of Large Volume Displacement” Keisuke Sano et al, and in EP 0 424 966A, to use a by-pass system of one form or another, such that the trap is used only when required and is not exposed to large volumes of exhaust gas above the temperature at which the zeolite trap material begins to degrade. HC traps in bypass arrangements may only reach 100-300° C., for example. Such HC traps incorporating a by-pass are now considered to be necessary to meet upcoming emission regulations.
In addition, “On-board diagnostics” systems (OBD) are now mandatory to show the driver of a vehicle when the TWC has failed or is operating at reduced efficiency. An effective method of carrying out OBD of a TWC is to measure the efficiency of the Oxygen Storage Component (OSC) of the TWC, and to trigger a failure signal if the efficiency falls below a predetermined value. The state-of-the-art OSCs incorporated in a TWC are required to have long term stability at gasoline exhaust gas temperatures of at least 850° C. For this reason, OSCs are generally highly stable ceria-zirconia mixed oxides, often with additional stabilization provided by doping the ceria-zirconia with an additional rare earth oxide such as the oxides of neodymium and/or lanthanum.